Officials said Castillo had served as CEO of the company Desarrollos Energeticos (DESA) -- which Caceres actively campaigned against over plans to build a hydroelectric dam -- at the time of the activist's slaying.

He was "responsible for providing logistics and other resources to the perpetrators," the statement said.

Caceres opposed DESA's plans to construct the dam across a river upon which indigenous communities depended.

Two masked gunmen fatally shot the activist at her home in La Esperanza, northwest of the capital Tegucigalpa on March 3, 2016. She was 44.

Her murder sparked international outrage and highlighted threats to Honduran activists.

At least eight other people have been arrested in connection with the homicide -- among them employees of DESA -- but Caceres's family has repeatedly demanded the masterminds be brought to justice.

In a statement issued after his arrest, the company said that "Castillo, like all members of DESA, are totally dissociated from the unfortunate incident that ended the life of Ms Berta Caceres."

"We respectfully request the immediate release of Mr David Castillo," read the statement.

DESA's dam project is currently suspended, but it has not been canceled.

- A "visionary" leader -

Caceres "taught us to defend the river, the land and the forest -- we are not going to surrender," said Paulina Gomez, a member of the Lenca indigenous community, as she rallied some 400 marchers over a loudspeaker late Friday along the main street of La Esperanza.

Demonstrators included Lenca members in western Honduras as well as relatives of the late activist.

More events honoring Caceres are planned for Saturday in La Esperanza, located 110 kilometers (63 miles) west of the capital Tegucigalpa.

The slain activist's mother, Austra Berta Flores, told AFP that Honduran Attorney General Oscar Chinchilla has rejected her demand to name an international commission to investigate the murder.

Flores also claimed that Castillo visited Caceres before the murder "to try to intimidate and bribe her" into halting her protest actions.

Caceres's daughter Olivia Zuniga, a member of the Honduran Congress, said she would introduce a measure to cancel the dam project as well as 48 other contracts to build dams in western Honduras.

The Honduras branch of the UN High Commissioner for Human Rights on Twitter said it "continues to express its support for the family" of Caceres and "efforts to seek the truth" on the anniversary of her death.

The Americas branch of the British charity Oxfam described Caceres in a tweet as "a visionary indigenous, environmental and feminist leader, adding: "we must also continue the struggle for justice."

Erika Guevara Rosas, Americas Director at the Human Rights group Amnesty International, said Friday that failure to resolve the Caceres case "sends a chilling message that human rights defenders can be killed with impunity if they dare question those with power in Honduras."

Another Rights group, Global Witness, named Honduras the deadliest country in the world for environmental human rights defenders.

Honduras has been rocked by political crisis for months, and dozens of people have been killed and hundreds jailed since President Juan Orlando Hernandez was declared the winner of the November 26 run-off election.

Supporters of defeated opposition candidate Salvador Nasralla accuse Hernandez -- who has the implicit backing of the United States -- of setting up a "military dictatorship," and insist that the election was stolen.